1. Field of the Invention
The present invention relates to a thin film for an anode of a lithium secondary battery and a manufacturing method thereof, and more particularly, to a thin film for an anode of a lithium secondary battery having improved charging/discharging cycle characteristics by using a multi-layered thin film as an anode active material layer, the multi-layered thin film formed by stacking a silver (Ag) layer and a silicon-metal (Si-M) layer having silicon dispersed in a base made from metal reacting with silicon while not reacting with lithium.
2. Description of the Related Art
According to recent remarkable growth of microelectronics industry and development of miniaturized, highly efficient electronic devices and miniature sensor devices, there is an increasing demand for sub-miniaturized, ultra-thin film batteries as a power source for driving these devices.
FIG. 1 is a schematic diagram of a conventional thin film battery.
Referring to FIG. 1, a thin film battery is basically constructed such that a cathode 12, an electrolyte 14, an anode 13 and a protective layer 15 are sequentially stacked on a current collector 11 in forms of thin films, and the overall thickness of the layered structure is approximately 10 μm. The thus-constructed conventional thin film battery has the following advantages.
Since the thin film battery is fabricated by depositing a cathode and an anode in the form of thin films, the battery can have a high current density. Also, since the cathode and the anode are formed in the form of thin films, the moving distance among ions is reduced to thus facilitate and promote ionic movement, thereby reducing the amounts of reactants. Since such thin film batteries can be easily manufactured in arbitrary shapes and sizes to be conformable to special purposes, they are very promising as main power sources for miniaturized electronic devices, micro-electromachanical systems (MEMS) and miniature sensor devices.
In particularly, since a thin film battery is manufactured by the same method as that of a semiconductor device, it can be mounted on a semiconductor chip together with an electronic circuit, thereby implementing a complementary metal oxide semiconductor (CMOS) memory chip using the thin film battery as a back-up power source. Also, an unused area of an electronic device can be minimized, thereby increasing the space utilization efficiency of the electronic device to a maximum. Further, since thin film batteries operating at various voltages and capacities can be realized by serial and parallel connections of unit cells through appropriate designs and etching steps, they can be widely used in a variety of applications.
Studies hitherto made on thin film batteries have concentrated on manufacture and evaluation of cathode thin films made of V2O5, LiCoO2 or LiMn2O4, and satisfactory results have been reported. Thin films for an anode for such thin film batteries are typically lithium thin films formed by deposition of a lithium metal.
However, the lithium metal has a low melting point of approximately 180° C. and is melted due to heat generated during soldering in the course of packaging, resulting in damage of a device. Also, since the lithium metal is highly reactive in the air, its managability is poor and a separate device for isolating the lithium metal from moisture and oxygen must be additionally installed. Thus, the use of a lithium metal has various problems to be widely put into practical use as an electrode material for power sources of super miniaturized electronic devices.
In addition to the lithium thin films, attempts at development of thin films for an anode made of silicon tin oxynitride (SITON), tin oxide (SnO2), or nitride, have been made. However, such attempts have not completely been unsuccessful. That is, these thin films for an anode have several problems in controlling irreversible reactions taking place during initial charging/discharging cycles.
In order to overcome the low charging/discharging cycle efficiency of lithium, research into lithium alloys has been carried out. Much attention has been paid to metals capable of forming lithium alloys, such as tin (Sn), silicon (Si) or aluminum (Al), as promising next generation anode active materials. While these anode active materials have good capacity characteristics at low operating voltages in contrast with lithium, a volumetric change in active material, encountered by insertion and release of lithium during charging/discharging cycles, results in a poor thin film structure for an anode and impediment to cycle characteristics, thereby reducing charge/discharge capacity. In particular, in the case of a thin film battery using a solid electrolyte, adhesion at an interface between an electrode and a current collector is considerably reduced, lowering battery performance. Thus, it is a critical issue to develop anode active material having no reduction in capacity due to irreversible reactions during insertion and release of lithium during the first charging/discharging cycle, as anode materials that can replace a lithium metal used in the prior art.